Immune adherence (IA) refers to the attachment of microbes, upon their exposure to serum, to blood cells. It is a fundamental process for initiating the destruction of an infectious organism and for promoting an immunological response. First described by experimental pathologists around the turn of the 20th century and then rediscovered in the 1950s, this phenomenon was eventually demonstrated to be dependent upon the coating of the microbe (antigen) with complement fragments (specifically, those of C3) and then recognition of the resulting complex by a C3 receptor on erythrocytes and leukocytes. Our goal is to continue our 20 year pursuit relative to elucidating the structure and function of this protein, termed complement receptor 1 (CR1, CD35, or the C3b/C4bIIA receptor), which binds and processes complement-bearing immune complexes. In the first specific aim, we propose to define the structure of the active sites of CR1. During the preceding grant period, the NMR derived structure of one of the two functional sites was obtained. This result will now be used to guide further mutagenesis aimed at defining the C3b and C4b binding face. The second specific aim will employ NMR, crystallography and other means to define regions of C3b and C4b that bind to the active sites and to determine the structure of the other major functional site of CR1. Our ultimate goal is to define the structure of the C3b and C4b interaction with CR1. Rosette formation between P. falciparum infected and uninfected erythrocytes, a laboratory marker of severe malarial infection, is mediated by CR1. We hypothesize that this adherence reaction is a pathologic process and that several structural variations in CR1, especially as expressed on erythrocytes, arose in response to this infection in order to inhibit this adherence reaction. In the third specific aim, we focus on 1) analyzing the interaction of a malaria adhesin protein with CR1 and 2) determining the functional consequences of these CR1 structural variations. Specific CR1 variations include allelic variants of CCP 25 which are common in Africans native to the malaria belt, and the CR1-like immune adherence receptors of non-human primates.